Wave washers are sold individually. (Thread cutters typically use 6-8 wave washers for proper function.) Wave washers work in tandem with sharp blades on APQS machines with a lower thread cutter to trim bobbin thread. The washers provide spring tension between the blades to effectively cut the bobbin thread as the puller arm brings the thread to the blades. Thread cutters will benefit from frequent cleaning between the thread cutter blades to prevent lint build-up. Use compressed or canned air to clean between the blades, and replace the wave washers if they become too flat to properly separate the cutter blades.